Obesity and its sequelae including diabetes are epidemic. Manipulating fat cells could lead to new therapies for obesity-associated disease. Peroxisome proliferator-activated receptors (PPARs) are involved in obesity and diabetes. One of these receptors, PPAR, is required for adipogenesis. Identification of endogenous PPAR ligands has been elusive. We have now identified an endogenous ligand for PPAR and developed an analogous strategy for identifying endogenous PPAR ligands. This work was predicated on the finding that inactivation of fatty acid synthase (FAS) in mouse liver produces a phenotype resembling PPAR deficiency that is rescued by pharmacologic activation of PPAR. Mass spectrometry techniques identified a lipid bound to PPAR that was FAS-dependent. This application focuses on obesity-resistant animals with FAS deficiency in adipose tissue, FASKOF (Fatty Acid Synthase KnockOut in Fat) mice. FAS-deficient embryonic fibroblasts have a phenotype resembling PPAR deficiency that is rescued by pharmacologic activation of PPAR. This project will test the hypothesis that in adipose tissue and its precursors, fatty acid synthase, an enzyme required for the process of de novo lipogenesis, mediates risk for obesity and metabolic disease in part by activating the nuclear receptor PPAR. The specific aims are: 1. To determine if mice with FAS inactivation in adipose tissue (FASKOF mice) are protected from diet- induced and genetic obesity and if pharmacologic activation of PPAR reverses protection. 2. To determine if the presence of FAS is required for PPAR activation and the normal adipocyte differentiation process. 3. To use mass spectrometry analyses to identify potential endogenous PPAR ligands by comparing lipids bound to PPAR isolated from FASKOF (FAS-deficient) and control (FAS-replete) cells. This project has the potential to help clarify how endogenous ligands are generated for a nuclear receptor implicated in the modulation of adiposity, glucose metabolism and inflammation.